1. Field of Invention
The present invention relates to an under-bump-metallurgy layer. More particularly, the present invention relates to an under-bump-metallurgy layer that can improve the mechanical strength of a barrier layer therein.
2. Description of Related Art
In this information-hungry society, electronic products are used almost everywhere to meet our demands for communication, business transactions, education, recreation and much more. The principle drivers behind all these electrical devices are specially designed integrated circuits. As electronic technologies continue to advance, increasingly complex, functionally powerful and highly personalized electronic products are produced. Rapid progress in design has also brought about the current trend of product miniaturization. Many types of high-density semiconductor packages are developed using flip-chip technique. Since a flip-chip packages utilize the bump on each contact pad of a chip to make direct electrical contact with a substrate, average circuit length is shorter than other types of packages connected through the wire bonding or the tape automated bonding (TAB) method. The shortened circuit length improves overall performance of a flip-chip package over other conventional packages. Furthermore, the backside of the chip in a flip-chip package may be exposed by design to increase heat dissipation. Because of these advantages, flip-chip techniques for fabricating packages are adopted by most semiconductor package producers.
FIG. 1 is a magnified cross-sectional view of a portion of a conventional flip-chip package structure. As shown in FIG. 1, the flip-chip structure 100 includes a silicon chip 100 and a plurality of welding bump structures 170 (only one is shown in FIG. 1). Each welding bump structure 170 comprises an under-bump-metallurgy (UBM) layer 142 and a bump 160. The chip 110 has an active surface 112. The active surface 112 of the chip 110 has a passivation layer 114 and at least one contact pad 116 thereon. The passivation layer 114 has at least one opening 118 that exposes the contact pad 116. The under-ball-metallurgy (UBM) layer 142 is formed on the contact pad 116 of the chip 110. The UBM layer 142 includes an adhesion layer 120, a barrier layer 130 and a wettable layer 140. The adhesion layer 120 sits directly on the contact pad 116, the barrier layer 130 is over the adhesion layer 120 and the wettable layer 140 is over the barrier layer 130. The adhesion layer 120 is made from a material such as titanium or aluminum, the barrier layer 130 is made from a material such as nickel-vanadium alloy and the wettable layer 140 is made from a material such as copper. The bump 160 sits on the wettable layer 140. The bump is made from a material such as lead-tin alloy.
In general, the aforementioned flip-chip package structure 100 has a thin wettable layer 140 of between 0.3 to 0.8 μm. Moreover, the copper in the wettable layer 140 may react quickly with the tin inside the bump 160. At the end of the copper-tin reaction, the tin within the bump 160 may further react with the nickel inside the barrier layer 130. Since the inter-metallic layer formed by the relatively slow reaction (more than 30 seconds) between tin and nickel is lumpy and discontinuous, ultimate contact with the adhesion layer 120 will be poor. Hence, the bump 160 may easily peel off from the upper surface of the chip 110.